Methods for preparing alkali cellulose and water-soluble cellulose ether

ABSTRACT

Provided is a method for preparing cellulose ether which is transparent as dissolved in water and has low water-insoluble content. More specifically, provided is a method for preparing alkali cellulose, comprising a contact step of bringing a pulp sheet having a sheet density of 0.60 g/ml or less or being formed from pine, or chips into which the pulp sheet has been converted, into contact with an alkali metal hydroxide solution at 5 to 70° C. for 10 to 600 seconds to obtain an alkali cellulose reaction mixture, and a drain step of draining the reaction mixture, wherein an amount of the alkali metal hydroxide solution to be used for the contact step is selected so that the alkali cellulose obtained by the drain step has a ratio of a weight of alkali metal hydroxide component determined by neutralization titration of the alkali cellulose to a weight of solid component in the pulp {(alkali metal hydroxide component)/(solid component in the pulp)} of 0.3 to 1.5. Also provided is a method for preparing water-soluble cellulose ether, comprising a step of reacting the resulting alkali cellulose with an etherifying agent.

CROSS-RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No.2007-337084; filed Dec. 27, 2007, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for preparing alkali celluloseand water-soluble cellulose ether having the low insoluble fibercontent.

2. Description of the Related Art

Water-soluble cellulose ether is produced by reacting cellulose having,in the molecule thereof, both a crystalline portion and anon-crystalline portion with an etherifying agent to convert thecrystalline portion into the non-crystalline portion and thereby makingthe cellulose ether soluble in water. It is said that crystallinity ofcellulose owes to a hydrogen bond between intramolecular hydroxyl groupsattributable to the skeleton structure of the cellulose molecule. Thishydrogen bond, which is firm, disturbs hydration with a water moleculein water and becomes a cause for making the cellulose water-insoluble.Cellulose ether is prepared by converting the cellulose into alkalicellulose with an aqueous solution of an alkali such as NaOH, therebybreaking its crystallinity; and reacting the alkali cellulose with anetherifying agent to substitute the hydroxyl group of the cellulose bythe etherifying agent. The resulting alkali cellulose however does notcompletely lose crystallinity. It is industrially difficult tosubstitute all the hydroxyl groups of the cellulose by raising thedegree of substitution for ether so that commercially availablecellulose ethers are water-soluble but have a water-insoluble portion.The water-insoluble portion sometimes has a fiber scale of pulp, whichis a raw material cellulose, exceeding even 1000 μm.

Water-soluble cellulose ether becomes tacky when dissolved in water sothat it is used for a thickener for transparent shampoos and rinses,hair styling agents, eye drops, detergents for contact lens and thelike. For example, methyl cellulose or hydroxypropyl cellulose, which iswater-soluble cellulose ether, has a hydrophilic group and a hydrophobicgroup in the molecule thereof and thereby shows interfacial activity. Itis therefore used as a suspension stabilizer in suspensionpolymerization of vinyl chloride or vinylidene chloride and becomesuseful also as a raw material for transparent plastic wrap for domesticuse. Products in such applications are desirably transparent. Unlesswater-soluble cellulose ether is water-soluble and transparent at themolecular level with regard to an aqueous solution of the water-solublecellulose ether, defective portions appear in the products and they maylead to inferior transparency or inferior function. An aqueous solutionof cellulose ether desirably has a high viscosity. Cellulose etherhaving a high viscosity has the higher insoluble fiber content thancellulose ether having a low viscosity so that it is thought to bedifficult to obtain a transparent product.

With a view to overcoming the above-described problems, Japanese PatentApplication Examined Publication 53-12954/1978 proposes a methodcomprising a step of allowing a raw material pulp to adsorb an aqueousalkali solution having a concentration of 15 to 75% by weight at 5 to80° C., and then pressing the resulting pulp within 10 seconds to removean extra portion of the aqueous alkali solution, a step of repeating theabove step to obtain the corresponding alkali cellulose, and a step ofreacting the alkali cellulose with an etherifying agent.

Japanese Patent Application Unexamined Publication No. 10-259201/1998proposes a method comprising steps of impregnating a pulp havingdichloromethane extract content of 0.07% by weight or less with sodiumhydroxide, pressing the resulting pulp to obtain the correspondingalkali cellulose, and then etherifying the alkali cellulose.

According to Japanese Patent Application Unexamined Publication2001-354701, cellulose ether is produced by a method comprising steps ofpulverizing a pulp sheet having a sheet density of 0.4 to 1.0 g/ml intopowders having an average particle size of 1000 μm or less, adding analkali to the powders to yield the corresponding alkali cellulose, andthen reacting the alkali cellulose with methyl chloride, propylene oxideand the like.

According to A. W. Anderson and R. W. Swinehart, Tappi, Vol. 39, No. 8,548-553, August, 1956, presented is a method for producing alkalicellulose, comprising a step of impregnating a pulp sheet having a sheetdensity of 0.47 to 1.17 g/ml in a bath containing an alkali solution for0.5 to 4.5 seconds.

According to U.S. Pat. No. 2,102,205, pulp is impregnated in an aqueoussolution of sodium hydroxide for 2 hours, and then pressed.

SUMMARY OF THE INVENTION

The present inventors have found that in the method according toJapanese Patent Application Examined Publication No. 53-12954/1978,adsorption/removal of an aqueous alkali solution is performed twice sothat the pulp swells with the solution and becomes fragile during thesecond adsorption/removal and as a result, troubles tend to occur andthe cellulose ether produced by this method does not have a satisfactoryquality; in the methods according to Japanese Patent ApplicationUnexamined Publication Nos. 10-259201/1998 and 2001-354701, distributionof the alkali tends to be uneven because of use of powdery pulp so thatcellulose ether having sufficiently low insoluble fiber content cannotbe obtained; and in the method according to A. W. Anderson and R. W.Swinehart, Tappi, Vol. 39, No. 8, 548-553, August, 1956, thedistribution of the alkali becomes uneven due to too short impregnationtime so that satisfactory cellulose ether cannot be produced. Theinventors have also found that the alkali cellulose produced by themethod according to U.S. Pat. No. 2,102,205 has an extremely high sodiumhydroxide/cellulose weight ratio of 3.0 so that an amount of sidereaction increases, and as a result such alkali cellulose is not suitedfor the preparation of cellulose ether.

Considering the above findings, the present invention aims at providinga method for preparing cellulose ether which is transparent as dissolvedin water and has low water-insoluble content.

The present inventors have carried out an extensive investigation with aview to overcoming the above-described problem. As a result, it has beenfound that cellulose ether which is transparent as dissolved in waterand has low water-insoluble content can be prepared by using, as a rawmaterial, alkali cellulose prepared by a method comprising steps ofbringing a pulp sheet having a certain sheet density or being formedfrom pine, or chips into which the pulp sheet has been converted, intocontact with an excess alkali metal hydroxide at a certain temperaturefor a certain period of time and then removing an extra portion of thealkali metal hydroxide, leading to the completion of the invention.

More specifically, the present invention provides a method for preparingalkali cellulose, comprising a contact step of bringing a pulp sheethaving a sheet density of 0.60 g/ml or less or being formed from pine,or chips into which the pulp sheet has been converted, into contact withan alkali metal hydroxide solution at 5 to 70° C. for 10 to 600 secondsto obtain an alkali cellulose reaction mixture, and a drain step ofdraining the reaction mixture, wherein an amount of the alkali metalhydroxide solution to be used for the contact step is selected so thatthe alkali cellulose obtained by the drain step has a ratio of a weightof alkali metal hydroxide component determined by neutralizationtitration of the alkali cellulose to a weight of solid component in thepulp {(alkali metal hydroxide component)/(solid component in the pulp)}of 0.3 to 1.5. The invention also provides a method for preparingwater-soluble cellulose ether, comprising a step of reacting the alkalicellulose with an etherifying agent.

According to the present invention, cellulose ether which is transparentas dissolved in water and has low water-insoluble content can beprepared.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter inwhich embodiments of the invention are provided with reference to theaccompanying drawings. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

The terminology used in the description of the invention herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used in the description ofthe invention and the appended claims, the singular forms “a”, “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

Hereinafter, preferred embodiments of the present invention will bedescribed. However, it is to be understood that the present invention isnot limited thereto.

The pulp sheet to be used in the present invention may be, for example,wood pulp or cotton linter pulp. Pulp derived from wood may beespecially preferred in order to obtain cellulose ether having lowinsoluble fiber content. As the wood, softwood such as pine, spruce andhemlock and hardwood such as eucalyptus and maple can be used. Of these,pulp using a pine as a raw material may be especially preferred. Pinesare plants in the genus Pinus. This pulp may use, as a raw material,wood belonging to the genus Pinus, for example, southern pines such asslash pine, loblolly pine, longleaf pine and shortleaf pine, andmaritime pine. The average length of fibers may be preferably from 2 to4 mm, average width of fibers may be preferably from 30 to 50 mm, andaverage thickness of cell membranes may be preferably from 3 to 5 mm.

The pulp sheet to be used in the present invention can have a sheetdensity of 0.60 g/ml or less, preferably 0.55 g/ml or less. When thesheet density is greater than 0.60 g/ml, it is difficult to preparecellulose ether having a low insoluble fiber content. Although noparticular limitation is imposed on the lower limit of the densityinsofar as the pulp is industrially available, it is typically 0.30 g/mlor greater. On the other hand, when the pulp sheet formed from pine isused according to the invention, it has no particular sheet densitylimitation.

The term “sheet density” of the pulp sheet means a weight per unitvolume of a single sheet and can be measured in accordance with themethod as described in International Standards ISO 534:1988 and ISO536:1995.

The basis weight of the pulp is measured in accordance with ISO536:1995. For example, the weight of each of 20 test pieces of about 30cm square having a humidity controlled by leaving them for 4 hours at23° C. and 500 relative humidity is measured and the basis weight iscalculated based on the following equation. The values for the basisweight of the 20 test pieces are averaged.

G=(M/A)×10000

wherein G represents the basis weight (g/m²) of each test piece, Mrepresents weight (g) of each test piece, and A represents an area (cm²)of each test piece.

Next, the thickness of the pulp is measured in accordance with ISO534:1988. For example, a micrometer is placed on a vibration-proofhorizontal surface. A test piece is inserted between the pressurizingsurfaces of the micrometer. A movable pressurizing surface is operatedat a rate of 3 mm/s or less. After confirming that the test piece isretained between the pressurizing surfaces, a value immediately afterstabilization is read for measurement of a thickness. The thickness ofeach test piece is measured at two positions, totally 40 positions. Thedensity is calculated based on the following equation and densities ofthe test pieces are averaged.

D=G/(T×1000)

wherein D represents the density (g/cm³), G represents the basis weight(g/m²) and T represents the thickness (mm).

The pulp sheet to be used in the invention may have a thickness ofpreferably from 0.1 to 5 mm, more preferably from 0.5 to 2.0 mm. Whenthe thickness is greater than 5 mm, the pulp sheet may be pressed withgreat difficulty. When the thickness is smaller than 0.1 mm, the pulpsheet may be handled with difficulty because the sheet is apt to bebroken during impregnation and press. The alpha cellulose content may bepreferably 90% by weight or greater. When the alpha cellulose content isless than 90% by weight, an alkali absorption rate may decrease.

In addition, according to the present invention, a dichloromethaneextract content of the pulp sheet may be preferably 0.1% by weight orless, more preferably 0.05% by weight or less. When the dichloromethaneextract content is higher than 0.1% by weight, cellulose ether havinglow insoluble fiber content may not be obtained.

The dichloromethane extract content can be measured by the method asdescribed in TAPPI T204, the standards of the organization dedicated topulp and paper. For example, in accordance with TAPPI T204, 150 ml ofdichloromethane and about 10 g of pulp are charged in a Soxhletextraction flask and at least 24 extraction cycles are performed over 4to 5 hours while adjusting a heating temperature to give a boiling rateenabling reflux of the solvent at least 6 times per hour. Afterextraction, the flask is removed from the extraction apparatus and theextract liquid in the flask is evaporated to become 20 to 25 ml. Theextract is then washed with a small amount of a solvent, transferred toa weighing dish, placed in a drier, dried at 105±3° C. for one hour,cooled in a desiccator, and weighed with an accuracy of 0.1 mg, andthus, “oven-dry weight of the extract” is determined. The “oven-dryweight of a blank residue” is also determined by the measurement using ablank composed only of a solvent and weight correction of the extract isperformed. The extract content is determined in accordance with thefollowing equation.

Extract content (%)={(oven-dry weight of extract)−(oven-dry weight ofblank residue)}/(oven-dry weight of pulp)×100

The “oven-dry weight of pulp” is determined by transferring pulp to aweighing dish, placing the dish in a drier, drying the pulp at 105±3° C.for 4 hours, cooling it in a desiccator and weighing the dried pulp withan accuracy of 0.1 mg.

Pulp having an intrinsic viscosity of preferably 300 ml/g or greater,more preferably 1000 ml/g or greater as measured in accordance withSCAN-CM 15:99, the standards of the organization dedicated to pulp andpaper, may be used. This is because highly viscous cellulose ether hasmarkedly high insoluble fiber content when prepared in the conventionalmanner. The intrinsic viscosity may be especially preferably 130 ml/g orgreater. The upper limit of the intrinsic viscosity may be that of pulptypically available and may be typically 2100 ml/g.

For example, 25 ml of distilled water is added to a pulp sample(provided in an amount to give [η]c of 3.0±0.1 which will be obtainedlater) in a vessel and then several pieces of copper wire are added.After a stopper is placed in the vessel, the vessel is shaken until thecomplete fracture of the pulp. Then, 25.0 ml of a copper ethylenediaminesolution is added to the vessel. After removal of air, the vessel issealed hermetically. The sample solution and a capillary viscometer areadjusted to 25.0° C. The sample solution is introduced into theviscometer. An efflux time t_(n) is measured and a viscosity ratioη_(rel) is calculated in accordance with the following equation:

η_(rel) =h×t _(n)

The symbol h is a viscometer constant as determined using a viscometerfor calibration, a viscometer for sample measurement and a glycerolsolution.

On a numerical table described in SCAN-CM 15:99, [η]c is read fromη_(rei). Separately, the concentration c (oven-dry pulp concentration)g/ml of the sample solution is calculated and a value obtained bydividing [η]c by c is designated as an intrinsic viscosity [η] ml/g.

As for the preparation of cellulose ether, a method described, forexample, in Japanese Patent Application Examined Publication No.53-12954/1978 may be especially preferred. According to this method,cellulose ether is obtained by impregnating a pulp sheet or pulp chipswith an excess alkali metal hydroxide solution, removing an extraportion of the alkali metal hydroxide solution by pressing the resultingpulp sheet or chips to prepare alkali cellulose, and then adding anetherifying agent to the alkali cellulose to cause reactiontherebetween. It is difficult to prepare cellulose ether having lowinsoluble fiber content by using, as raw material, alkali celluloseprepared without impregnating the pulp with an excess alkali metalhydroxide solution.

The pulp chips to be used in the invention have chip shapes and can beobtained by cutting the pulp sheet. Although no limitation is imposed onthe production method of pulp chips, existing cutters such as a slittercutter can be used. Cutters capable of cutting the pulp successively maybe advantageous from the viewpoint of an investment cost.

The pulp chips may usually have a side of preferably 2 to 100 mm, morepreferably 3 to 50 mm. When the side is less than 2 mm, it may bedifficult to prepare uniform alkali cellulose because cellulose fibersmay be damaged so that an alkali metal hydroxide solution may notsmoothly penetrate into the fibers. When the side is greater than 100mm, it may be difficult to handle the pulp chips, especially to chargethem in an impregnating apparatus, move them inside of the apparatus andcharge them in a separator.

In the invention, the pulp sheet or the chips into which the pulp sheethas been converted is brought into contact with an excess alkali metalhydroxide and then, an extra portion of the alkali metal hydroxide isremoved. For example, a method comprising steps of impregnating the pulpsheet or the chips with an excess alkali metal hydroxide solution andthen draining to remove an extra portion of the alkali metal hydroxidesolution can be used. Examples of the method may include a methodcomprising steps of impregnating a pulp sheet in a bath containing analkali metal hydroxide solution and then pressing the resulting sheetwith a roller or another apparatus under pressure; and a methodcomprising steps of impregnating pulp chips in a bath containing analkali metal hydroxide solution and then pressing the resulting pulpchips by centrifugal separation or another mechanical method. The term“excess alkali metal hydroxide” means an alkali metal hydroxide in anamount exceeding a weight ratio of (alkali metal hydroxidesolution)/(cellulose) in the alkali cellulose to be provided for theetherifying reaction in the last step and it allows a weight ratio of analkali metal hydroxide solution to a solid component in pulp (alkalimetal hydroxide solution)/(solid component in pulp) to fall within arange of preferably from 3 to 5,000, more preferably from 10 to 200,still more preferably from 20 to 60. When the weight ratio is less than3, the alkali metal hydroxide and the pulp may be brought into contactwith difficulty. Although no upper limit is determined, a very excessalkali metal hydroxide solution requires excessive equipment so thatfrom the economic viewpoint, it may be typically about 5000.

The reason why the weight of an alkali metal hydroxide solution is usedinstead of the weight of an alkali metal hydroxide is that it isimportant for the pulp to physically come into uniform contact with(impregnated in) the alkali metal hydroxide solution, avoiding theexistence of pulp not in contact (wetted) with the alkali metalhydroxide solution because of an excessively small amount of the alkalimetal hydroxide solution.

Although the alkali metal hydroxide solution to be used in the inventionis not limited insofar as alkali cellulose is produced by using it, anaqueous solution of sodium hydroxide or potassium hydroxide may bepreferably selected from the economic viewpoint. The concentration ofthe alkali metal hydroxide solution may be preferably from 23 to 60% byweight, especially preferably from 35 to 55% by weight. The alkali metalhydroxide solution may be preferably an aqueous solution, but it may bea solution in an alcohol such as ethanol or a mixed solution in awater-soluble alcohol and water.

The pulp is brought into contact with the alkali metal hydroxidesolution at preferably from 5 to 70° C., more preferably from 15 to 60°C. When the temperature is less than 5° C., the alkali metal hydroxidesolution has high viscosity so that an absorption rate at which the pulpabsorbs the solution may decrease. This may not be preferred from theviewpoint of the productivity. When the temperature is higher than 70°C., the alkali metal hydroxide solution has low viscosity so that anabsorption rate at which the pulp absorbs the solution may increase andthe composition of the resulting alkali cellulose may vary widely. Thismay not be preferred from the viewpoint of quality.

The pulp is brought into contact with the excess alkali metal hydroxidefor a period of time from 10 to 600 seconds, preferably from 15 to 120seconds. When the contact time is less than 10 seconds, the compositionof the resulting alkali cellulose may vary widely so that it may not bepreferred from the viewpoint of quality. When the contact time is morethan 600 seconds, the absorption amount of the alkali metal hydroxideinto the pulp may increase excessively, which may lead to a failure toproduce alkali cellulose having a desired composition.

According to the invention, an amount of the alkali metal hydroxidesolution to be used for the contact step is selected so that a ratio ofthe weight of alkali metal hydroxide component as determined byneutralization titration of the alkali cellulose obtained by the drainstep to the weight of solid component in the pulp {(alkali metalhydroxide component)/(solid component in pulp)} falls within a range of0.3 to 1.5, preferably 0.65 to 1.30, more preferably 0.90 to 1.30.

Since the pulp serving as a starting material is typically composed ofcellulose and water, the solid component in the pulp is cellulose. Whenthe above-described weight ratio is from 0.3 to 1.5, the resultingcellulose ether can have high transparency.

The solid component in the pulp may include, in addition to cellulosewhich is a main component, organic matters such as hemicellulose, ligninand resins, and inorganic matters such as Si and Fe components.

With regard to the alkali cellulose obtained by the drain step, theweight ratio of (alkali metal hydroxide component)/(solid component inthe pulp) can be determined by the following neutralization titrationmethod.

A total weight of a cake of the alkali cellulose obtained by the drainstep is measured. First, 4.00 g of the cake of the alkali celluloseobtained by the drain step is sampled and the percent by weight (wt %)of the alkali metal hydroxide contained in the cake is determined byneutralization titration (0.5 mol/L H₂SO₄, indicator: phenolphthalein).A blank test is also performed in a similar manner.

Wt % of alkali metal hydroxide=(normality factor)×[{amount (ml) of H₂SO₄added dropwise}−{amount (ml) of H₂SO₄ added dropwise in blank test}]

In the above equation, the molecular weight of sodium hydroxide is setat 40.

If the wt % of the alkali metal hydroxide is determined, the “alkalimetal hydroxide component” in the total amount of the cake of the alkalicellulose obtained by the drain step can be determined.

The “solid component in the pulp” can be determined, for example, bysampling about 2 g of pulp, drying it at 105° C. for 4 hours, andfinding a percentage (wt %) of the weight of the dried pulp in theweight of the sampled pulp.

The weight ratio of (alkali metal hydroxide component)/(solid componentin the pulp) for the alkali cellulose obtained by the drain stepapproximates to a weight ratio of (alkali metal hydroxidecomponent)/(alkali cellulose component in a narrow sense) for the alkalicellulose obtained by the drain step as described below.

The weight ratio of (alkali metal hydroxide component)/(alkali cellulosecomponent in a narrow sense) can be determined in accordance with thefollowing equation by using wt % of the alkali metal hydroxide containedin the cake obtained by the drain step.

(wt of alkali metal hydroxide)/(wt of alkali cellulose in a narrowsense)=(wt % of alkali metal hydroxide)÷[{100−(wt % of alkali metalhydroxide)/(B/100)}×(S/100)]

wherein B represents the concentration (wt %) of the alkali metalhydroxide solution and S represents the concentration (wt %) of thesolid component in the pulp.

In the equation, {100−(wt % of alkali metal hydroxide)/(B/100)} means wt% of the component contained in the cake but other than the alkali metalhydroxide solution. Assuming that alkali cellulose in a narrow sense ispresent at a similar wt % to the wt % of the solid component in thepulp, S/100 is multiplied to obtain the wt % of the alkali cellulose.

The term “alkali cellulose in a narrow sense” means a concept narrowerthan the alkali cellulose obtained by the drain step and containing thealkali metal hydroxide, and means the alkali cellulose itself afterremoval of the alkali metal hydroxide solution.

The alkali cellulose thus obtained can be cut into an adequate size, forexample, into chips and supplied to an etherification reactor. Theetherification reactor may be preferably a reactor where anetherification reaction takes place while grinding the alkali celluloseby a mechanical force until the chips lose their shapes. Anetherification reactor having, inside thereof, a stirring mechanism istherefore preferred. Examples of the reactor may include a plough typeshovel blade mixer such as a ploughshare mixer. Prior to introducing thealkali cellulose into the etherification reactor, it can be ground inadvance using another apparatus having a stirring mechanism inside or agrinder such as a cutter mill.

After the pulp is brought into contact with the alkali metal hydroxide,homogenization and mercerization reaction proceed, whereby aging occurs.When their contact time is too short, however, the aging does not occursufficiently. As a result, insoluble fiber content of the celluloseether produced by etherification of the alkali cellulose increases. Itmay be therefore preferred that 50% by weight or less, more preferably30% by weight or less, of the alkali cellulose to be reacted with theetherifying agent is within 60 minutes after the pulp is brought intocontact with the excess alkali metal hydroxide. When the percentage ofthe alkali cellulose which is within 60 minutes is greater than 50% byweight, the proportion of the alkali cellulose which has agedinsufficiently may raise and the cellulose ether thus obtained may haveincreased insoluble fiber content.

Examples of the cellulose ether which can be produced by using theresulting alkali cellulose as a starting material may include alkylcelluloses such as water-soluble methyl cellulose (MC); hydroxyalkylcelluloses such as hydroxypropyl cellulose (HPC) and hydroxyethylcellulose (HEC); hydroxyalkylalkyl celluloses such ashydroxypropylmethyl cellulose (HPMC), hydroxyethylmethyl cellulose(HEMC) and hydroxyethylethyl cellulose (HEEC); and carboxymethylcellulose and carboxymethyl cellulose sodium (CMC—Na).

Examples of the alkyl cellulose may include methyl cellulose having amethoxyl group (DS) of from 1.0 to 2.2 and ethyl cellulose having anethoxyl group (DS) of from 2.0 to 2.6.

Examples of the hydroxyalkyl cellulose may include hydroxyethylcellulose having a hydroxyethoxyl group (MS) of from 0.05 to 3.0 andhydroxypropyl cellulose having a hydroxypropoxyl group (MS) of from 0.05to 3.3.

Examples of the hydroxyalkylalkyl cellulose may includehydroxyethylmethyl cellulose having a methoxyl group (DS) of from 1.0 to2.2 and a hydroxyethoxyl group (MS) of from 0.1 to 0.6;hydroxypropylmethyl cellulose having a methoxyl group (DS) of from 1.0to 2.2 and a hydroxypropoxyl group (MS) of from 0.1 to 0.6; andhydroxyethylethyl cellulose having an ethoxyl group (DS) of 1.0 to 2.2and a hydroxyethoxyl group (MS) of from 0.1 to 0.6.

Examples also may include carboxymethyl cellulose having acarboxymethoxyl group (DS) of from 0.2 to 2.2.

It should be noted that alkyl substitution is expressed by DS andhydroxyalkyl substitution is expressed by MS. They each means an averagenumber of moles of an etherifying agent attached to a glucose unit andcan be calculated from the results obtained in accordance with themeasurement method of the Japanese Pharmacopoeia.

Examples of the etherifying agent may include alkyl halides such asmethyl chloride and ethyl chloride; alkylene oxides such as ethyleneoxide and propylene oxide; and monochloroacetic acid.

The viscosity of a 2% by weight aqueous solution of the cellulose etherat 20° C. may be preferably from 2 to 200000 mPa·s, more preferably from50 to 100000 mPa·s.

When cellulose ether is prepared with insufficient degree ofsubstitution or without carrying out uniform substitution, manyinsoluble fibrous substances having a size of about 16 to about 200 μminevitably remain when the cellulose ether is dissolved in water. Thenumber of these insoluble fibrous substances can be counted in thefollowing manner. The cellulose ether is dissolved in an aqueouselectrolyte solution for coulter counter, ISOTON II (product of Coulter)in a temperature-controlled bath at 25° C. so as to obtain a 0.1% byweight aqueous solution and then, the number of insoluble fibers havinga size of 16 μm or greater but not greater than 200 μm present in 2 mlof the resulting solution is counted with an aperture tube of 400 μm indiameter by using a Coulter Counter TA II or a Multisizer manufacturedby Coulter. The cellulose ether containing preferably 100 or less, morepreferably 60 or less insoluble fibers, as measured in the above manner,is excellent. When the number of insoluble fibers is too low to bemeasured, it is possible to use a high concentration solution for themeasurement as needed and convert the result into that in terms of a0.1% by weight aqueous solution.

The light transmittance of a 2% by weight aqueous solution of thewater-soluble cellulose ether of the invention at 30° C. may bepreferably 96% or greater, especially preferably 97% or greater whenmeasured using a PC-50 type electrical calorimeter, a cell length of 20mm, and visible light.

According to the invention, the following cellulose ether may bepreferably used from the viewpoint improving its solubility. Aftershaking 100 g of the cellulose ether powder for 30 minutes in a ro-tapsieve shaker Model No. 429 manufactured by Kansai Wire Netting Co., Ltd.by using a standard sieve No. 100 (with openings of 150 μm) asprescribed by JIS Z8801 at a shaking frequency of 200 cycles/min, atapping number of 156 taps/min and a stroke of 50 mm, 25% by weight orless of powder residue remains on the sieve.

The invention will hereinafter be described in further detail byExamples and Comparative Examples. It should not be construed that theinvention is limited to or by them.

EXAMPLE 1

Alkali cellulose was obtained by impregnating a pulp sheet A derivedfrom a pine (maritime pine) and having an intrinsic viscosity of 1300ml/g, a sheet density of 0.60 g/ml and a dichloromethane extract contentof 0.10% by weight with a 49% by weight aqueous NaOH solution of 50° C.for 12 seconds and then pressing the resulting sheet to remove an extraportion of the 49% by weight aqueous NaOH solution. During theimpregnation step, a weight ratio of (49% weight aqueous NaOHsolution)/(solid component in the pulp) was 100. A weight ratio of (NaOHcomponent of alkali cellulose thus obtained)/(solid component in thepulp) was 1.25.

The 20 kg of the resulting alkali cellulose was placed in aninternal-stirring type pressure-resistant reactor. After vacuuming, 11kg of methyl chloride and 2.7 kg of propylene oxide were added theretoto carry out the reaction while grinding. The reaction mixture was thenwashed, dried and ground to yield hydroxypropylmethyl cellulose. Whenthe addition of methyl chloride and propylene oxide was started, 45% byweight of the alkali cellulose was within 60 minutes after the pulp wasbrought into contact with the excess alkali.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. A 2% by weight aqueous solution of the hydroxypropylmethylcellulose had a viscosity at 20° C. of 28000 mPa·s and a lighttransmittance at 30° C. of 98.0% which was measured using a PC-50 typeelectrical calorimeter, a cell length of 20 mm and visible light. Thenumber of insoluble fibers having a size of 16 μm or greater but notgreater than 200 μm was 80. The results are shown in Table 1.

EXAMPLE 2

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 1 except that when the addition of methyl chloride and propyleneoxide was started, 55% by weight of the alkali cellulose was within 60minutes after the pulp was brought into contact with the excess alkali.The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 3

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 1 except that a rotational reactor having no grinding functionwas used. The hydroxypropylmethyl cellulose thus obtained had a degreeof methoxyl substitution (DS) of 1.90 and a degree of hydroxypropoxylsubstitution (MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 4

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 1 except that a pulp sheet B derived from a pine (maritime pine)and having an intrinsic viscosity of 1300 ml/g, a sheet density of 0.60g/ml and a dichloromethane extract content of 0.12% by weight was usedin the place of the pulp sheet A. The hydroxypropylmethyl cellulose thusobtained had a degree of methoxyl substitution (DS) of 1.90 and a degreeof hydroxypropoxyl substitution (MS) of 0.24. Evaluation results areshown in Table 1.

EXAMPLE 5

Alkali cellulose was obtained in a similar manner to in Example 1 exceptthat a pulp sheet C derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.55 g/ml and adichloromethane extract content of 0.05% by weight was impregnated witha 49% by weight aqueous NaOH solution of 15° C. for 600 seconds. Aweight ratio of (NaOH component of the resulting alkalicellulose)/(solid component in the pulp) was 1.25. Using the resultingalkali cellulose as a raw material, hydroxypropylmethyl cellulose wasobtained as in Example 1.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 6

Alkali cellulose was obtained in a similar manner to in Example 1 exceptthat a pulp sheet D derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.50 g/ml and adichloromethane extract content of 0.05% by weight was impregnated witha 49% by weight aqueous NaOH solution of 35° C. for 40 seconds. A weightratio of (NaOH component of the alkali cellulose thus obtained)/(solidcomponent in the pulp) was 1.25. Using the resulting alkali cellulose asa raw material, hydroxypropylmethyl cellulose was obtained as in Example1.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 7

A pulp sheet D derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.50 g/ml and adichloromethane extract content of 0.05% by weight was converted intochips of 10 mm square. After the resulting pulp in the chip form wasimpregnated for 30 seconds with a 49% by weight aqueous NaOH solution of35° C., it was pressed using a rotating basket having a centrifugaleffect of 500 to remove an extra portion of the 49% by weight aqueousNaOH solution, whereby alkali cellulose was obtained. In theimpregnation step, a weight ratio of (49 wt % aqueous NaOHsolution)/(solid component in the pulp) was 15.

A weight ratio of (NaOH component of the alkali cellulose thusobtained)/(solid component in the pulp) was 1.25. Using the resultingalkali cellulose as a raw material, hydroxypropylmethyl cellulose wasobtained as in Example 1. The hydroxypropylmethyl cellulose thusobtained had a degree of methoxyl substitution (DS) of 1.90 and a degreeof hydroxypropoxyl substitution (MS) of 0.24. Evaluation results areshown in Table 1.

EXAMPLE 8

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 7 except for the use of a pulp sheet E derived from spruce andhaving an intrinsic viscosity of 1300 ml/g, a sheet density of 0.50 g/mland a dichloromethane extract content of 0.05% by weight. Thehydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 9

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 7 except for the use of a pulp sheet F derived from a pine(maritime pine) and having an intrinsic viscosity of 300 ml/g, a sheetdensity of 0.50 g/ml and a dichloromethane extract content of 0.05% byweight. The hydroxypropylmethyl cellulose thus obtained had a degree ofmethoxyl substitution (DS) of 1.90 and a degree of hydroxypropoxylsubstitution (MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 10

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 7 except for the use of a pulp sheet G derived from a pine(maritime pine) and having an intrinsic viscosity of 1000 ml/g, a sheetdensity of 0.50 g/ml and a dichloromethane extract content of 0.05% byweight. The hydroxypropylmethyl cellulose thus obtained had a degree ofmethoxyl substitution (DS) of 1.90 and a degree of hydroxypropoxylsubstitution (MS) of 0.24. Evaluation results are shown in Table 1.

EXAMPLE 11

Hydroxypropylmethyl cellulose was obtained in a similar manner to inExample 7 except for the use of a pulp sheet H derived from a pine(maritime pine) and having an intrinsic viscosity of 2000 ml/g, a sheetdensity of 0.50 g/ml and a dichloromethane extract content of 0.05% byweight. The hydroxypropylmethyl cellulose thus obtained had a degree ofmethoxyl substitution (DS) of 1.90 and a degree of hydroxypropoxylsubstitution (MS) of 0.24. Evaluation results are shown in Table 1.

COMPARATIVE EXAMPLE 1

Alkali cellulose was obtained in a similar manner to in Example 1 exceptfor the use of a pulp sheet I derived from a pine (maritime pine) andhaving an intrinsic viscosity of 1300 ml/g, a sheet density of 0.62 g/mland a dichloromethane extract content of 0.04% by weight. A weight ratioof (NaOH component of the alkali cellulose thus obtained)/(solidcomponent in pulp) was 1.25. Using the resulting alkali cellulose as araw material, hydroxypropylmethyl cellulose was obtained as in Example1.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

COMPARATIVE EXAMPLE 2

A pulp sheet D derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.50 g/ml and adichloromethane extract content of 0.05% by weight was impregnated witha 49% by weight aqueous NaOH solution of 55° C. for 8 seconds and thenpressed to remove an extra portion of the 49% by weight aqueous NaOHsolution, whereby alkali cellulose was obtained. A weight ratio of (NaOHcomponent of the alkali cellulose thus obtained)/(solid component inpulp) was 1.25. Using the resulting alkali cellulose as a raw material,hydroxypropylmethyl cellulose was obtained as in Example 1.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

COMPARATIVE EXAMPLE 3

A pulp sheet D derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.50 g/ml and adichloromethane extract content of 0.5% by weight was ground with aknife mill to yield powdery pulp having an average particle size of 200μm. The resulting powdery pulp was impregnated with a 49% by weightaqueous NaOH solution of 35° C. for 30 seconds and then pressed using arotating basket having a centrifugal effect of 500 to remove an extraportion of the 49% by weight aqueous NaOH solution, whereby alkalicellulose was obtained. During the impregnation step, a weight ratio of(49 wt % aqueous NaOH solution)/(solid component in pulp) was 15. Aweight ratio of (NaOH component of the resulting alkalicellulose)/(solid component in pulp) was 1.25. Using the resultingalkali cellulose as a raw material, hydroxypropylmethyl cellulose wasobtained as in Example 1.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

COMPARATIVE EXAMPLE 4

The pulp sheet D derived from a pine (maritime pine) and having anintrinsic viscosity of 1300 ml/g, a sheet density of 0.50 ml/g and adichloromethane extract content of 0.05% by weight was ground with aknife mill to obtain a powdery pulp having an average particle size of200 μm. The 8.0 kg, on a dry basis, of the resulting powdery pulp wasplaced in an internal stirring type pressure-resistant reactor. Aftervacuuming, 20.4 kg of 49% by weight NaOH of 40° C. was sprayed understirring to prepare alkali cellulose having a weight ratio of (NaOHcomponent)/(solid component in pulp) of 1.25. Following the preparation,11 kg of methyl chloride and 2.7 kg of propylene oxide were added tocarry out a reaction. The reaction mixture was then washed, dried andground to yield hydroxypropylmethyl cellulose.

The hydroxypropylmethyl cellulose thus obtained had a degree of methoxylsubstitution (DS) of 1.90 and a degree of hydroxypropoxyl substitution(MS) of 0.24. Evaluation results are shown in Table 1.

TABLE 1 portion pulp of alkali presence viscosity CH₂Cl₂ cellulose of of2 wt % number sheet extract impregnation within 60 mins grinding aqueouslight of derived density content temp. time after contact duringsolution transmit. insoluble type from viscosity form (g/ml) (%) (° C.)(sec.) (%) reaction (mPa · s) (%) fibers Example 1 A pine 1300 sheet0.60 0.10 50 12 45 present 28000 98.0 80 Example 2 A pine 1300 sheet0.60 0.10 50 12 55 present 28500 97.0 88 Example 3 A pine 1300 sheet0.60 0.10 50 12 45 absent 28000 97.0 87 Example 4 B pine 1300 sheet 0.600.12 50 12 45 present 27500 97.0 90 Example 5 C pine 1300 sheet 0.550.05 15 600 45 present 28500 98.5 60 Example 6 D pine 1300 sheet 0.500.05 35 40 45 present 28100 98.5 55 Example 7 D pine 1300 chips 0.500.05 35 30 45 present 30000 98.5 50 Example 8 E spruce 1300 chips 0.500.05 35 30 45 present 29500 97.5 60 Example 9 F pine 400 chips 0.50 0.0535 30 45 present 350 98.5 52 Example10 G pine 1000 chips 0.50 0.05 35 3045 present 8500 98.5 53 Example11 H pine 2000 chips 0.50 0.05 35 30 45present 110000 98.5 65 Comp. Ex. 1 I pine 1300 sheet 0.62 0.04 50 12 45present 28000 94.0 180 Comp. Ex. 2 D pine 1300 sheet 0.50 0.05 55 8 45present 28000 94.0 170 Comp. Ex. 3 D pine 1300 powder 0.50 0.05 35 30 45— 30000 94.0 200 Comp. Ex. 4 D pine 1300 powder 0.50 0.05 — — — — 4500091.0 600

1. A method for preparing alkali cellulose, comprising: a contact stepof bringing a pulp sheet having a sheet density of 0.60 g/ml or less, orchips into which the pulp sheet has been converted, into contact with analkali metal hydroxide solution at 5 to 70° C. for 10 to 600 seconds toobtain an alkali cellulose reaction mixture, and a drain step ofdraining the reaction mixture, wherein an amount of the alkali metalhydroxide solution to be used for the contact step is selected so thatthe alkali cellulose obtained by the drain step has a ratio of a weightof alkali metal hydroxide component determined by neutralizationtitration of the alkali cellulose to a weight of solid component in thepulp {(alkali metal hydroxide component)/(solid component in the pulp)}of 0.3 to 1.5.
 2. A method for preparing alkali cellulose, comprising: acontact step of bringing a pulp sheet obtained by using pine as a rawmaterial or chips into which the pulp sheet has been converted, intocontact with an alkali metal hydroxide solution at 5 to 70° C. for 10 to600 seconds to obtain an alkali cellulose reaction mixture, and a drainstep of draining the reaction mixture, wherein an amount of the alkalimetal hydroxide solution to be used for the contact step is selected sothat the alkali cellulose obtained by the drain step has a ratio of aweight of alkali metal hydroxide component determined by neutralizationtitration of the alkali cellulose to a weight of a solid component inthe pulp {(alkali metal hydroxide component)/(solid component in thepulp)} of 0.3 to 1.5.
 3. The method for preparing alkali celluloseaccording to claim 1, wherein the pulp sheet has a dichloromethaneextract content of 0.10% by weight or less.
 4. A method for preparingwater-soluble cellulose ether, comprising a step of reacting alkalicellulose prepared by the method according to claim 1 with anetherifying agent.
 5. The method for preparing water-soluble celluloseether according to claim 4, wherein 50% by weight or less of the alkalicellulose to be reacted with the etherifying agent has been within 60minutes after the pulp is brought into contact with the alkali metalhydroxide solution.
 6. The method for preparing water-soluble celluloseether according to claim 4, wherein the alkali cellulose to be reactedwith the etherifying agent is in form of chips and is reacted with theetherifying agent after or while being ground.
 7. The method forpreparing alkali cellulose according to claim 2, wherein the pulp sheethas a dichloromethane extract content of 0.10% by weight or less.
 8. Amethod for preparing water-soluble cellulose ether, comprising a step ofreacting alkali cellulose prepared by the method according to claim 2with an etherifying agent.
 9. The method for preparing water-solublecellulose ether according to claim 8, wherein 50% by weight or less ofthe alkali cellulose to be reacted with the etherifying agent has beenwithin 60 minutes after the pulp is brought into contact with the alkalimetal hydroxide solution.
 10. The method for preparing water-solublecellulose ether according to claim 9, wherein the alkali cellulose to bereacted with the etherifying agent is in form of chips and is reactedwith the etherifying agent after or while being ground.